1. Field of the Invention
This invention relates generally to the fabrication of magnetic heads and more particularly to a method of manufacture that eliminates or significantly reduces the occurrence of electrostatic discharge damage to elements of the head during manufacture of the head disk assembly (HDA).
2. Description of the Related Art
A magnetic data recording hard disk drive employs a plurality of electromagnetic transducers, a typical one of the prior art being schematically shown in FIG. 1. In a state-of-the-art disk drive, each transducer (1) further consists of a read-head (2) and a write-head (4). The read-head (2) is typically a sensor whose operation is based on the giant magnetoresistive (GMR) effect and it is extremely sensitive to electrostatic discharge (ESD). The write-head is typically an inductive coil (coil cross-sections shown as (6)), which is fairly immune to ESD. The read head is typically protected from stray electromagnetic fields during operation by an upper shield (8) and an under-shield (10). A first dielectric layer, D1 (11) insulates the under-shield from the GMR element (13), which is the multi-layered sensor that utilizes the GMR effect. A second dielectric layer, D2 (15) insulates the GMR element from the upper shield (8). The shields, dielectric layers and GMR film are sequentially formed on a substrate, which will form a slider (discussed below). The combined read/write-head (transducer) is typically encapsulated in an alumina deposit (12) and conducting leads (not shown) pass through the encapsulation and are connected to terminals (usually gold) (14) on its surface.
As is further shown in prior art FIG. 1, each transducer (1) is mounted on a wear-resistant ceramic carrier (20), commonly called a slider. The write head's magnetic core (18) and the read head's GMR sensor element (13), emerge at a surface of the encapsulating alumina deposit (12) which is co-planar with the surface of the slider (22) called its air-bearing surface (ABS). The surface of the slider is commonly protected by a highly wear-resistant carbon overcoat (COC) (21). As is shown further in prior art FIG. 2, each slider (20) is mounted on a stainless steel suspension (23) using a conductive adhesive (24), the combination of (20) and (23) forming a head-gimbals assembly (HGA). In a typical manufacturing process, each suspension carries a flexible circuit (30), such as a cable or printed wiring ribbon. The circuit can be schematically depicted as having a plurality of conductive traces (32), of which only one is visible here, and an insulating layer (34). The flexible circuit is firmly attached to the suspension at the end facing the slider, where it is electrically connected to the terminals of the transducer (36). The other end of the flexible circuit will ultimately be connected to a signal processing circuit as discussed below.
Referring next to prior art FIG. 3, a plurality of HGA's (only one being shown in the figure) is attached to a conductive rigid frame (40), to form a head-suspension assembly (HSA). The HSA is then merged (not shown) with a stack of magnetic recording disks to form a head-disk assembly (HDA). At some point in the manufacturing process, electrical connections (41) will be made from each transducer to the HDA signal processing circuit (42). This connection involves the free ends of the flexible circuits whose other ends have been connected to the GMR head. Although FIG. 3 shows the connection as completed, typically by solder re-flow such as is taught by Ainslie et al. (U.S. Pat. No. 4,761,699), there is a period of time in the manufacturing process wherein the freely suspended traces (32) are not yet connected but have been routed to a position whereat connection can be made. The process of routing these trace “tails” into final position is a process wherein tribocharges (33) that have formed will flow and electrostatic discharge (ESD) damage is likely to occur. Referring now to prior art FIG. 4, there is shown a schematic drawing of two of the insulation covered traces (32), and their surrounding insulation (34). The two traces shown represent the connections to the GMR read-head (the write-head connections are not shown) portion of the transducer (1) and are shown as being connected to the terminals at (14). As a result of unavoidable contacts between the insulation and surrounding elements of the fabrication, free electric charges (tribocharges) (46) are generated on the insulation (shown here as positive, but they are of arbitrary sign).
When the free ends of the traces (48) are grounded, either inadvertently or by design (indicated symbolically by a switch), the charge flows through the read-head (shown by the arrow (50)) and ESD damage occurs. To prevent such ESD damage, the two terminals may be shorted at the tails of the traces (the short is not shown, but it would be at (48)). There is also an option of shorting all four traces together, two for the read-head and two for the write-head. This approach has two disadvantages. First, the shunt used to create the short (at (48)) is distant from the read head connection (at (44)). Thus, the characteristic impedance of the transmission line (the two traces) is dominant over the shunt resistance and the shunt is rendered ineffective for dealing with a brief transient event such as the ESD. Second, the shunt must ultimately be removed before the leads can be connected to the HDA circuit, which is not a simple process and must also occur without damage or contamination to the fabrication. In addition, the process of removing the shunt can itself lead to an ESD event.
One alternative to prevent ESD damage from accumulated tribocharge is to subject the fabrication to ionized airflow. This method is ineffective because the time constant for tribocharge to accumulate to damaging levels is on the order of microseconds, whereas the decay time of static charge accumulations in ionized airflow is on the order of seconds. In short, the effect of ionized airflow is too slow to dissipate the tribocharge before an adverse ESD occurs.
Another alternative is to coat or replace the insulator with dissipative material. This adds to the expense of the process. In addition, charge dissipation requires conductivity, so a dissipative “insulator” is slightly conductive, and this can add to cross-talk between the leads to read and write heads. The need to prevent cross-talk prohibits the use of dissipative insulators that could reduce the time constant for the decay of tribocharge sufficiently to prevent more than between 5% and 50% of the ESD damage to the head-gimbals assembly.
An improvement can be achieved by coating the insulator with a volatile dissipative material, such as isopropyl alcohol. This material will evaporate before actual electrical testing of the assembly, so that cross-talk problems do not arise, yet the required increased dissipation of tribocharge will occur during assembly. A similar approach is to cause water to condense on the insulator, either by blowing hot and humid air on it or by refrigerating the work piece. These improvements are difficult to achieve because of environmental and operational concerns.
Still another approach is to cover up the ends of the trace tails until they are properly positioned for soldering (at (41) of FIG. 3) so that accidental contact with the surroundings becomes impossible. This, however, is difficult due to the small size of the leads, which are approximately 1 mm wide by 0.1 mm thick. A better alternative is to cover up the conductive rigid frame ((40) in FIG. 3), to eliminate a major region of inadvertent contact. However, this does not prevent the tails from contacting each other and it renders the process of aligning the tails for soldering more difficult.
The greatest challenge in all of the above ESD control methods is to remove tribocharge on the flexible connector circuit rapidly and without adverse side-effects. The presence of tribocharge is independent of the GMR head design. However, as the recording technology continues to evolve and the storage density on recorded media continues to increase, GMR heads will continue to shrink in size and become more susceptible to ESD damage. Competitive GMR heads are routinely ranked by original equipment manufacturer (OEM) customers according to their threshold against ESD damage.
A search of prior art references discloses, in detail, several methods that have been applied to the shielding of magnetic heads. Jurisch et al. (U.S. Pat. No. 4,972,286) teaches a method for grounding a thin film magnetic read/write head by means of a direct connection between the base substrate of the head and a pole of the write head or its core. The connection is made by a conducting stud that extends through a base coat layer that is sandwiched between the core and the base substrate. This mechanism is proposed as a means of reducing noise during the operation of the read head. Schwarz et al. (U.S. Pat. No. 4,800,454) teaches a method for eliminating arcing between the pole tip of an inductive write head and the flyer (or, in our terminology, the slider) on which it is mounted. The method teaches the dissipation of accumulated static charges by means of a conductive connection between the transducer and the flyer. According to a pertinent embodiment, a low resistance, low current capacity bleeder wire is connected between the windings and the flyer body. After the winding is connected to the circuitry, a large current is passed through the winding to destroy the bleeder. Takada (U.S. Pat. No. 6,331,924) teaches a method of forming an MR head with a capacitor connected in parallel between the head and a substrate wherein the substrate serves as one of the capacitor plates. The object of the method is to provide an alternate path for an ESD current other than through the head itself. Watanuki (U.S. Pat. No. 6,267,903) teaches a method of head fabrication wherein elements of the head are formed on portions of a substrate which has first been formed an enclosing electrically conductive film. According to the method, static charges produced during the manufacturing process of the various element sections are prevented from passing through the element itself. None of these methods is applicable to the ESD damage problem encountered in the fabrication of heads of the design type discussed above. The method of Jurisch would not be applicable to such a head design and, moreover, the method of Jurisch is directed to protecting the head during actual use. The method of Schwarz is primarily directed to the protection of the coil windings in the inductive write head portion of the GMR head. A burnable wire within the sensitive read-head would not be practical. Takada's approach is to reduce the potential of the magnetic head by a capacitative connection to the substrate. This approach would be inadequate for the product design discussed above. Finally, Watanuki's method assumes the flow of static charges is within the plane of the element formation. Surrounding the element region with a conducting moat would not be practical for the present production method, nor would it eliminate ESD damage that due to tribocharges that pass through the layers of the GMR read-head.
The present invention proposes a significant improvement over the methods of the prior art, particularly when applied to the manufacturing process of a head designed as discussed herein with reference to FIGS. 1-4a. While the insulator ((34) in FIG. 2) cannot be grounded effectively, it is not necessary to remove tribocharge from the insulator. As long as the transducer ((1) in FIG. 1) remains grounded continuously, its potential will be unaffected by the accumulation or dissipation of tribocharge on the insulator or on any nearby conductors. Therefore, in the case of a grounded transducer, it is completely safe for its terminals ((14) in FIG. 1), or any conductors connected to them, to contact any other grounded conductor during assembly. It is, therefore, possible to design a HGA with a low ESD damage threshold in conventional tests, yet which is free of ESD damage during the assembly process.
It is further noted that the primary ground path of the read-head runs through the carbon overcoat (COC) ((21) of FIG. 1), the slider substrate ((20) of FIG. 1), the conductive adhesive ((24) of FIG. 2), to the stainless-steel suspension ((23) of FIG. 2). The suspension is connected to a fixture or a conductive rigid frame, either of which is easily grounded. The resistance, R, of the ground path is dominated by the resistance of the COC, which is typically on the order of mega-ohms. Since the capacitance, C, of the read-head is on the order of pico-farads, the time constant, τ=RC, of static discharge is significantly greater than a microsecond. Therefore, the potential of the read-head is strongly affected by tribocharge.
A novel solution is to connect the read-head to the slider substrate through a much smaller resistance to reduce the time constant. For simplicity, a direct connection with negligible resistance is acceptable. Although we have cited prior art that teaches a connection between the write-head core to the slider substrate (eg. Jurisch and Schwarz cited above), and although connections between a read-head and its shields are also taught (related patent application HT99-019, fully incorporated herein by reference), no direct connection between the read-head and the slider substrate is taught in the prior art. Superficially, it would seem that this approach is destined to fail. Connecting an additional conducting lead to the read-head should make the read-head even more susceptible to ESD damage, since an additional lead offers an additional source of inadvertent contact with external grounded elements of the system. Indeed, all conventional tests indicate that such an additional lead lowers the threshold for ESD damage to the read-head. However, it is also the case that it is easier to control electrostatic potentials within the environment of the read-head than it is to control the potentials induced by tribocharges on the insulators of the flexible circuit. As long as the read-head remains continuously grounded, a low threshold for ESD damage due to contacts between the charged flexible circuit and its surroundings does not present an appreciable threat. Similarly, when conductors within the surroundings are grounded, there is also a very low threat of ESD damage to the read-head. Thus, a sensible strategy to pursue in suppressing ESD damage to the read head during construction of the head-suspension assembly is to maintain the read-head at a very low potential. The more usual approaches of avoiding accidental contacts or strengthening the sensor itself are much less effective.